This application is related to Applicants"" concurrently filed U.S. application Ser. No. 09/598,486, entitled xe2x80x9cProcess For The Production Of An Isomer Of Xylenes In Three Stages: Separation, Isomerization In The Presence Of A Catalyst Based On An EUO Zeolite And Transalkylationxe2x80x9d, based on French Application 99/07.967 filed Jun. 22, 1999 and U.S. Ser. No. 09/598,651, entitled xe2x80x9cProduction Of A Xylene Isomer In Three Stages: Separation, Isomerization With A Catalyst With An EUO Zeolite Base And Transalkylation With Recycling Of C10-Aromatic Compoundsxe2x80x9d, based on French Application 99/07.966 filed Jun. 22, 1999.
The invention relates to a process for the production of at least one isomer of xylenes, from a C8-aromatic fraction, whereby said process comprises the scheme of a separation stage by adsorption in a simulated moving bed of paraxylene or of a mixture of orthoxylene and metaxylene or metaxylene or ethylbenzene, and an isomerization stage of the fraction that is low in the desired isomer.
The prior art is illustrated by Patent Application FR-768 724 of the applicant.
Although the principle of the combination of an adsorption stage in a simulated moving bed and an isomerization stage, as well as the one that preferably consists in carrying out, in particular in the case of paraxylene, at least one crystallization stage, is already described in the prior art, the fact of using in the isomerization process an isomerization catalyst that comprises at least one EUO-structural-type zeolite, for example the EU-1 zeolite, and at least one element of group VIII of the periodic table makes it possible to improve, surprisingly enough, the productivity of the scheme and to reduce its losses. In a preferred embodiment of the invention, an adsorbent that incorporates a zeolitized binder that has a high adsorption capacity will be used in the adsorption stage. According to a variant of the invention, in the adsorption process, the injection of small amounts of water with the desorbent makes it possible to reduce the solvent level. Finally, according to another variant of the invention, a recrystallization stage on the mother liquor is carried out in the crystallization process.
The combination of this variety of variants makes it possible to improve, surprisingly enough, the productivity of the scheme and to reduce its losses by synergy effects.
The production of a specific isomer of the xylenes is an important petrochemical process in the synthesis of the polyesters, used in particular in the fabric manufacturing industry. It is then important to be able to synthesize the desired isomer, preferably the paraxylene with maximum purity. Several techniques for separating isomers have been developed. Thus, the separation of the isomers can be done by adsorption, for example in a zeolitic sieve, that delivers a fraction that is very high in paraxylene and a fraction that is low in paraxylene and therefore high in particular in orthoxylene and metaxylene, in the presence of an elution solvent. Since the composition of the aromatic feedstocks with eight carbon atoms varies broadly, however, according to their origin, with the para and ortho isomer content generally coming close to 50%, a single process does not make it possible to maximize the production of the desired isomer, such as, for example, paraxylene. It then is necessary to combine an adsorption stage of the feedstock followed by an isomerization stage of this fraction that is low in desired isomer as is described in, for example, Patent GB 1,420,796. Patent EP 531 191 of the applicant describes a process for the production of paraxylene by treatment in an adsorption zone that is followed by at least one stage for crystallization of the adsorbed paraxylene, whereby the raffinate that is low in paraxylene is sent into an isomerization zone.
In U.S. Pat. No. 5,401,476 of the applicant, the described combination is a scheme of:
1. a process for separation of paraxylene in a simulated moving bed preferably with a small number of beds where an effort is made to obtain primarily a very high productivity and a small solvent level at the expense of the purity, a first fraction that is high in paraxylene and toluene and a second fraction that is high in other C8-aromatic isomers and that contains toluene are obtained,
2. a first distilling column that can separate paraxylene from toluene,
3. a second distilling column that can separate the other C8-aromatic isomers from toluene,
4. a crystallization that makes it possible, starting from paraxylene with a purity of between 75% and 98%, to produce paraxylene of commercial purity (at least 99.5%) and a mother liquor that is recycled in part in stage 15) of an isomerization that treats the mixture of other C8-aromatic isomers to produce paraxylene in thermodynamic equilibrium with other C8-aromatic isomers, then after elimination of the light components that are produced during the isomerization, recycling in stage 1.
The three main stages that are described in the prior art were greatly improved by the applicant; as a result, by a synergy effect, the combination became much more productive and the relative values of the different flows have very little to do with the values that are provided in the examples of U.S. Pat. No. 5,401,476.
The adsorption stage was improved with regard to the ratio of feedstock flow rate to solvent flow rate as described in French Patent 2,757,507 by the injection of water with a controlled content: instead of a solvent level on the order of 1.35/1, the injection of water either in the desorbent stream or in the feedstock stream to obtain a weighted water mean on the outlets of about 80 ppm in the case of an adsorbent with an X zeolite base that is exchanged with barium and about 10 ppm in the case of a Y zeolite that is exchanged with barium and potassium made it possible to reduce, for example, the solvent level to 1/1 in the first case and 1.2/1 in the second case.
The adsorption stage has also been improved with regard to its monitoring and its stability: U.S. Pat. Nos. 5,578,215 and 5,578,216 teach how to compensate the volume differences from bed to bed and in particular the one that is created by the recycling pump or by the production of a shorter bed for the stage that is integral with the recycling pump, or by the desynchronization of the switching times of the various flows to compensate for the delays or advances of the elution fronts. In contrast, French Patent 2,762,793 of the applicant shows how, starting from at least one analysis point of the internal concentration profile, it is possible to monitor the inside ratios of the liquid flow rates with solid flow rates to remain permanently at peak performance levels despite the inevitable fluctuations of flow rate and composition of the feedstock that is to be separated.
The adsorbent itself was greatly improved by using a particular binder that it is possible to transform at least in part into a zeolite after the shaping and either before or after the ion exchange. In this way, the adsorption capacity is increased by at least 15% (FR 99/02151). In contrast, by using a very particular X zeolite, with an Si/Al ratio that is close to 1, it is possible to increase the adsorption capacity even more (FR-A-2 767 524).
In French Patent Application FR 2729660, the applicant showed more specifically how a second crystallization stage that is carried out on the mother liquor that exits the first crystallization makes it possible to increase the yield of the crystallization and to reduce significantly the recycling flow rate of mother liquor to the adsorption stage.
For the isomerization stage, the catalyst that is used in the isomerization reactions is generally the mordenite that is mixed with other zeolites, such as the ZSM-5 zeolite, as described in U.S. Pat. Nos. 4,467,129; 4,482,773 and EP 138 617 B. Other catalysts have a mordenite base and have been described in, for example, U.S. Pat. Nos. 4,723,051; 4,665,258 and FR 2,477,903.
Catalysts for isomerization of the C8-aromatic fractions with a base of EUO-structural-type zeolites, i.e., the EU-1, TPZ3 and ZSM-50 zeolites, were described by the applicant in Patent Applications FR-A- 2 772 642 and FR-A- 2 772 752, incorporated as references.
The object of the invention is to eliminate the drawbacks of the prior art by proposing an optimized solution in which the productivity is increased and the losses are reduced. Another object of the invention is to describe a hybrid process for the production and separation of paraxylene with a productivity that is greater than the one that is obtained with the processes of the prior art and whose losses are reduced with the use of a more selective isomerization catalyst and preferably with reduced recycling owing to the implementation of a set of special means described below. Within the meaning of this description, hybrid process refers to a process that comprises at least one crystallization stage that is downstream from the adsorption zone.
The process according to this invention makes it possible, surprisingly enough, to obtain yields of paraxylene that are much higher than those of the prior art with improved performance levels in activity and selectivity during the isomerization stage, which further brings about a reduction of the recycling volumes as well as a reduction in size of the recovery zone of the paraxylene. Actually, the process according to this invention makes it possible to limit the losses during the isomerization stage, with the use of a catalyst that comprises an EUO-structural-type zeolite. In addition, the stability of the isomerization catalyst is improved relative to the catalysts of the prior art. Also, the various improvements that are made in the separation zone make it possible to increase the productivity of the process significantly.
The invention relates to a process for the conversion of hydrocarbons and for the production of at least one isomer of xylenes that is selected from among orthoxylene, paraxylene and metaxylene with a yield of desired isomer that is improved relative to the processes of the prior art with the use in the isomerization zone of a catalyst with an EUO-structural type zeolite base and at least one metal of group VIII of the periodic table. This process can preferably use an adsorbent that incorporates a zeolitized binder in the separation zone. According to another implementation of the process, it is possible to inject small amounts of water with the desorbent into the adsorption zone. This process preferably uses a recrystallization stage on the mother liquor in the crystallization zone that most often comprises an arrangement of several stages, under rigid temperature and pressure conditions that make it possible to obtain optimal operation of the unit.
More specifically, the process for production of at least one xylene isomer of this invention in general comprises the following stages:
a) In at least one simulated moving-bed adsorption zone, a feedstock that contains aromatic compounds with eight carbon atoms, i.e., metaxylene, paraxylene, ethylbenzene and orthoxylene, is brought into contact continuously with a zeolitic adsorbent bed in the presence of a suitable desorption solvent, under adsorption conditions such that a first fraction that contains solvent and that is high in desired isomer and a second fraction that is low in desired isomer and that comprises the majority of the other isomers and solvent are obtained. The first fraction will contain, for example, paraxylene with a purity of between 75 and 99.9%. The second fraction will then contain metaxylene, ethylbenzene and optionally orthoxylene,
b) the first fraction is distilled to separate the solvent, on the one hand, and the desired isomer, on the other hand,
c) the second fraction that is low in desired isomer is distilled, and the solvent, on the one hand, and the majority of the other isomers, on the other hand, are recovered,
d) the fraction that contains the majority of the other isomers recovered in stage c) is isomerized under suitable conditions in the presence of hydrogen in an isomerization zone, and an isomerate is recovered that is preferably at least in part recycled to stage a) after having generally eliminated the light compounds (with a boiling point that is less than 80xc2x0 C.), and fraction 80-135xc2x0 C. that is recycled, for example, at the inlet of the isomerization zone, whereby said process is characterized in that the isomerization reaction that is described in stage d) is implemented in the presence of a catalyst that comprises at least one EUO structural-type zeolite and at least one element of group VIII of the periodic table.
According to a preferred embodiment of the process according to this invention, it is possible to use at least one crystallization zone downstream from the adsorption zone. Thus, in the case where the desired isomer is paraxylene, at least one crystallization of the paraxylene of stage b) is initiated in at least one crystallization zone at a temperature of between +10xc2x0 C. and xe2x88x9225xc2x0 C., and, on the one hand, a mother liquor is obtained that can be recycled at least in part to stage a) and, on the other hand, paraxylene crystals that are saturated with mother liquor are obtained.
According to a preferred embodiment of the process according to this invention, it is possible to wash in a washing zone with a suitable washing solvent the crystals of the isomer that is desired, and these crystals are recovered. In the case where the desired isomer is paraxylene, the paraxylene crystals with a very high degree of purity, or generally at least 99.6% and preferably at least 99.8%, are recovered.
Thus, the catalyst that is used in the isomerization stage comprises at least one EUO zeolite, i.e., the EU-1, TPZ-3 and ZSM-50 zeolites.
The EUO-structural-type EU-1 zeolite, already described in the prior art, has a monodimensional microporous network, whose pore diameter is 4.1xc3x975.7 xc3x85 (1 xc3x85=1 angstrom=1.10xe2x88x9210 m) (xe2x80x9cAtlas of Zeolites Structure Types,xe2x80x9d W. M. Meier and D. H. Olson, 4th Edition, 1996). In contrast, N. A. Briscoe et al. taught in an article of the journal Zeolites (1988, 8, 74) that these monodimensional channels have side pockets with a depth of 8.1 and a diameter of 6.8xc3x975.8 xc3x85. The synthesis method of the EU-1 zeolite and its physico-chemical characteristics were described in Patent EP-42 226.
U.S. Pat. No. 4,640,829 relates to the ZSM-50 zeolite, which has the EUO-structural type according to the xe2x80x9cAtlas of Zeolites Structure Types,xe2x80x9d W. M. Meier and D. H. Olson, 4th Edition, 1996.
Patent Application EP-51 318 relates to the TPZ-3 zeolite, which has the EUO-structural type according to the xe2x80x9cAtlas of Zeolites Structure Types,xe2x80x9d W. M. Meier and D. H. Olson, 4th Edition, 1996.
In a preferred embodiment, this invention is also characterized in that the adsorbent comprises an X zeolite or a Y zeolite that is shaped with a binder such as a clay, for example, kaolin, that can be transformed into zeolite under special conditions of temperature, pressure and pH. After the transformation of at least 50% of the binder into zeolite, an exchange, for example, with barium ions or with strontium ions in the case of the X zeolite and, for example, with potassium ions and then barium in the case of the Y zeolite, is carried out.
According to a preferred embodiment, this invention is also characterized in that the adsorbent can have a higher grain size than in the prior art, and the adsorption temperature is a little higher. Actually, the adsorbent that is described in particular in the examples of U.S. Pat. No. 5,401,476 has a grain size of a spherical shape of 0.3 to 0.5 mm of diameter, which causes significant pressure drops. With a larger spherical-shaped grain size that is 0.4.to 0.8 mm in diameter, centered on 0.65 mm of diameter and a slightly higher adsorption temperature (160 to 170xc2x0 C.) to compensate for a more difficult material transfer, a pressure drop of about 2.5 times less per unit of length is obtained, and it is possible to use support beams of the distributor panels, whereby distributor panels and a ring have to meet fewer mechanical constraints, i.e., in terms of thickness and reduced cost. In contrast, the consumption of electricity of the recycling pump is also greatly reduced.
Still according to a preferred embodiment, this invention is also characterized in that it is possible to inject water into the adsorption zone, preferably with desorbent, to monitor the water content on the weighted mean of the flow rates of extract and raffinate, whereby this weighted mean depends, of course, on the type of zeolite that is used. It is also possible to reduce the solvent level relative to the feedstock.
According to another preferred embodiment of this invention (hybrid process), it is possible to use a unit of adsorbent beds that are placed inside a single column whose number usually varies from 9 to 15 according to the composition of the feedstock. For a feedstock that is easy to treat, comprising 24%, for example, of paraxylene and on the order of 4% of ethylbenzene, the number of beds is, for example, 10; for an average feedstock, for example, that comprises 22% of paraxylene and 10% of ethylbenzene, the number of beds is, for example, 12; finally for the most difficult feedstocks, for example, 17% of paraxylene and 30% of ethylbenzene, the number of beds is, for example, 15.
For the adsorption process that does not comprise (a) final crystallization stage(s), usually a set of adsorbent beds, placed inside two or more columns, whose number usually varies from 16 to 30 (24 beds, for example, for the production of paraxylene of 30 beds, for example, for the co-production of paraxylene and metaxylene) are used.
In a particular implementation of the invention, the first or the last bed of each column (depending on the valve system that allows the introduction or the draw-off of fluids) can have a small volume to compensate approximately the volume of the recycling loop: this compensation is such that all of the non-selective volumes of this bed added to the volume of the recycling loop is approximately equal to the non-selective volume of an intermediate bed. Once the unit is produced, the calculation of the amounts of adsorbent charged bed by bed makes it possible to compensate for which each of the flows is connected to each of the beds, in each case because of non-selective volumes of each of the beds. This correction technique is only possible, of course, provided that one all-or-nothing valve is used per bed and per flow. Once the unit is operational, on-line analyses are carried out continuously (by a Raman spectrometer, for example, as described in U.S. Pat. No. 5,569,808) or intermittently by a vapor phase chromatograph on the recycling loop. By knowing, on the one hand, the concentration profile and, on the other hand, the internal flow rates and the mean switching time, ratios of the liquid flow rate s to the solid flow rate or else the eluted volumes in each zone are calculated during a period that is used as a control variable for maximizing the purity and the yield continuously.
The separation zone comprises at least one adsorption zone in which is adsorbed the majority of the desired isomer or the majority of undesirable isomers. In an implementation of the invention, the separation zone makes it possible to recover paraxylene, i.e., paraxylene is adsorbed and recovered as an extract. other implementations of the invention make it possible to recover metaxylene and orthoxylene based on the selected adsorbent. The adsorption zone operates in a simulated moving bed and comprises at least one zeolitic adsorbent bed that operates in the presence of a suitable desorption solvent and under adsorption conditions such that there is obtained, in the case where an attempt is made to recover paraxylene, a first fraction that contains solvent, metaxylene, ethylbenzene, and orthoxylene, and a second fraction essentially of paraxylene and solvent. The paraxylene that is obtained generally has a purity of between 75 and 98% in the case of the hybrid process and 99.6 to 99.9% in the case of the process without crystallization. The first fraction is distilled to recover the paraxylene, on the one hand, and the solvent, on the other hand, and the second fraction is distilled to separate the solvent, on the one hand, and the mixture of metaxylene, orthoxylene and ethylbenzene, on the other hand. The second fraction is then sent into the xylene isomerization zone. The solvent can be recycled at the inlet of the separation zone.
The simulated moving bed can be a simulated countercurrent bed or a simulated co-current bed. The elution solvent or desorption solvent is selected, for example, from among toluene or paradiethylbenzene. This list is nonlimiting, and other solvents such as methyl-tert-butyl ether (MTBE) or diisopropyl ether (DIPE) can also be used.
The adsorbents that make it possible to adsorb the paraxylene selectively comprise at least one zeolite that is selected from among the X and Y zeolites of which most often the exchangeable sites are occupied by alkaline or alkaline-earth cations, such as, for example, potassium and barium.
The adsorbents that make it possible to adsorb the metaxylene and the orthoxylene selectively are, for example, the X or Y zeolites that are exchanged with, for example, at least one of the following metals: Li, Na, Be, Mg, Ca, Sr, Mn, Cd, Cu., Ni.
The xylene separation zone can preferably comprise at least: one crystallization zone downstream from the adsorption zone. Thus, for example, the paraxylene that is recovered after distillation of said second fraction is sent into at least one crystallization zone at a temperature of between, for example, +10 and xe2x88x9225xc2x0 C., and, on the one hand, a mother liquor is obtained that is preferably recycled to the adsorption zone, and, on the other hand, paraxylene crystals that are saturated with mother liquor are obtained, then it is washed with a suitable washing solvent to obtain paraxylene crystals with a very high degree of purity, or generally purity of greater than 996% and preferably greater than 99.8%.
A preferred two-stage crystallization process was described by the applicant in Patent Applications WO 96/20907, WO 96/20908 and WO 96/22262.
As solvent for washing crystals, it is possible to use, for example, n-pentane, water, purified paraxylene or toluene. The same solvent is preferably used for desorption and for washing crystals, such as, for example, toluene, or else purified paraxylene to avoid having to redistill the last traces of washing solvent.
The first distilled fraction that is recovered after the xylene separation stage that comprises orthoxylene, metaxylene and ethylbenzene is treated in an isomerization zone. Isomerization stage c) is implemented in the presence of a catalyst that comprises an EUO-structural-type zeolite, for the EU-1 zeolite. The EUO-structural-type zeolite is at least in part in acid form and comprises silicon and at least one element T that is selected from the group that is formed by aluminum, iron, gallium and boron, preferably aluminum and boron, with an overall Si/T atomic ratio that is greater than 5.
The EUO-structural-type zeolite, for the EU-1 zeolite, in the catalyst according to the invention, can be at least in part, preferably virtually totally, in acid form, i.e., in hydrogen form (II+), whereby the sodium content is preferably such that the Na/T atomic ratio is less than 0.5, preferably less than 0.1, even more preferably less than 0.02.
The catalyst also comprises at least one matrix that comprises at least one compound that is selected from the group that is formed by clays, magnesia, aluminas, silicas, titanium oxide, boron oxide, zirconia, aluminum phosphates, titanium phosphates, zirconium phosphates and silica-aluminas. The matrix is preferably alumina. The catalyst comprises at least one element of group VIII of the periodic table, preferably selected from among platinum and palladium, and it can also contain optionally at least one metal that is selected from among the metals of groups IIIA and IVA, preferably selected from among tin and indium and optionally sulfur.
The isomerization catalyst can comprise by weight relative to the total catalyst mass:
1 to 90% by weight of at least one EUO-structural-type zeolite, preferably 3 to 60% and even more preferably 4 to 40%,
0.01 to 2% by weight of at least one metal of group VIII, preferably 0.05 to 1%,
optionally 0.01 to 2% of at least one additional element that is selected from groups IIIA and IVA of the periodic table, preferably 0.05 to 1%,
optionally sulfur,
a binder that ensures the make-up by weight to 100% of catalyst.
The catalyst that is used in the process according to the invention can be prepared by any method that is known to ones skilled in the art and in particular by those that are described in the prior art that relate to the catalysts that contain at least one EUO-structural-type zeolite and in particular the EU1, ZSM-50 and TPZ-3 zeolite.
The catalyst is preferably prepared as described in the patent application of the applicant FR-A-2 772 642. Thus, the catalyst preferably has a dispersion of the metal of group VIII that is between 50 and 100%, and more preferably between 60 and 100% and even more preferably between 70 and 100%, a macroscopic distribution coefficient of said metal of group VIII of between 0.7 and 1.3. The catalyst is preferably shaped in the form of balls or extrudates and has a mechanical resistance such that the bed crushing value is higher than 0.7 MPa, preferably between 0.8 and 1.2.
The isomerization zone is usually operated at a temperature of about 300xc2x0 C. to 500xc2x0 C., preferably about 320xc2x0 C. to 450xc2x0 C. and even more preferably about 350xc2x0 C. to 420xc2x0 C., at a partial hydrogen pressure of about 0.3 to 1.5 MPa, preferably about 0.4 to 1.2 MPa, at a total pressure of about 0.45 to 1.9 MPa, preferably about 0.6 to 1.5 MPa, at a PPH (feedstock weight/catalyst weight/hour) of about 0.25 hxe2x88x921 to 30 hxe2x88x921, preferably about 1 to 10 hxe2x88x921, and very often 2 hxe2x88x921 to 6 hxe2x88x921. The hydrogen that is introduced for the production of the isomerization can be recycled in said isomerization zone.
In a particular implementation of the isomerization stage, the isomerization zone can comprise a recycling as described in the patent application of the applicant, FR-A-2 777 275. The process then comprises at least one distillation zone downstream from the isomerization zone to recover, after the fraction that comprises the light compounds (with a boiling point less than 80xc2x0 C.) is eliminated, a fraction that contains a majority of the aromatic compounds containing at least eight carbon atoms per molecule and that is sent into the xylene separation zone and so as to recover a fraction that comprises compounds with a boiling point of about 80xc2x0 C. to 135xc2x0 C. and more particularly at least one of the compounds that are selected from the group that consists of naphthenes with eight carbon atoms per molecule, the paraffins with eight carbon atoms per molecule, benzene and toluene, at least one of the compounds of said fraction, isolated from the entire fraction by treatment in at least one distillation zone, able to be recycled at the inlet of the isomerization zone. The percentage by weight of recycled compounds relative to the total feedstock that enters into the isomerization zone is between 0.01 and 20%. Actually, surprisingly enough, the fact of recycling at least one compound with a boiling point of between 80xc2x0 C. and 135xc2x0 C. makes it possible to decrease the parasitic reactions of the isomerization which, combined with the performance levels of the catalyst that is used within the scope of this invention, ensures considerable savings for the process. It is preferably possible to recycle the naphthenes with eight carbon atoms in the isomerization zone, whereby the other compounds of the fraction with a boiling point of between 80xc2x0 C. and 135xc2x0 C. such as toluene and paraffins can be recovered at this level of the process.
The output effluent of the isomerization zone that comprises the three isomers of the xylenes in a ratio that is essentially close to the one of the thermodynamic equilibrium is then either partly or totally recycled in the xylene separation zone after an optional treatment with earth (WO-96/20 907).
According to a preferred embodiment of the invention, a catalyst will be used that comprises an EUO-structural-type zeolite whose crystal size is smaller than 5 micrometers (xcexcm), often less than 0.5 xcexcm, and most often less than 0.2 xcexcm. These crystals or crystallizates are often at least in part grouped in aggregates that have a grain size such that the value of Dv,90 is less than or equal to 500 xcexcm, often less than 400 xcexcm, most often less than 200 xcexcm, and even more preferably less than or equal to 50 xcexcm. The size of the aggregates is determined by grain size with laser diffraction. This measurement is taken on the zeolite powder that is suspended in water. After a first measurement, the suspension is subjected to ultrasound for thirty seconds, then a new measurement is taken. The ultrasound that is used is characterized by a power of 50 W and a frequency of 50 kHz. This procedure is repeated until the result no longer varies (at +5%). The volume-defined size distribution of the aggregates is calculated starting from light signals that are collected by detectors and with Fraunhofer""s theory. Dv,X is defined as being the diameter of the equivalent sphere such that X% by volume of the aggregates has a size that is less than said diameter. These characteristics will be obtained directly during the synthesis of the zeolite and/or by any method that makes it possible to reduce the size of the aggregates, such as, for example, post-synthesis grinding or else a suitable kneading before shaping.